In a conventionally known ball screw, a ball rolling passage is jointly defined by a thread groove formed on the outer circumferential surface of a screw shaft and a thread groove formed on the inner circumferential surface of a nut, and a plurality of balls (rigid balls) are received in this ball rolling passage. The ball screw is typically used for converting a rotational motion of an electric motor into a linear motion with a high transmission efficiency and a high precision. During the operation of the ball screw, it is necessary for the balls received in the ball rolling passage to circulate. A typical arrangement for circulating the balls consists of an end deflector system in which a ball return passage for circulating the balls is formed on the side of the nut, and each end of the ball return passage is fitted with an end deflector (circulation member) for scooping up the balls from the ball rolling passage to the ball return passage and returning the balls from the ball return passage back to the ball rolling passage.
The known structures based on the end deflector system or other systems for circulating the balls include the arrangement where an end deflector is fitted into a receiving hole formed on each axial end of the nut from an axial direction (See Patent Document 1) and the arrangement where a deflector (side deflector) is fitted into each of a pair of receiving holes formed in axially intermediate points of the nut (peripheral wall) from a radial direction (See Patent Document 2).